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										00-面试前准备
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										01.简历
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										01-Java基础
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										01. 注解
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										02. 泛型
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										03. JavaNIO
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										04. 反射和动态代理
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										02-Java并发
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										01. 多线程基础
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										02. CAS
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										03. synchronized
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										04. AQS和ReentrantLock
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										05. 线程池
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										06. ThreadLocal
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										07. CountDownLatch和CyclicBarrier
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										08. 为什么需要Java内存模型
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										09. 深入浅出Java内存模型
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										03-Java容器
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										01. List集合
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										02. Map集合
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										04-Java虚拟机
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										01. Java编译到执行的过程
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										02. 双亲委派机制
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										03. JVM内存结构
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										04. 垃圾回收机制
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										05. CMS垃圾收集器
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										06. G1垃圾收集器
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										07. JVM调优
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										05-Spring
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										01. SpringMVC
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										02. Spring基础
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										03. SpringBean生命周期
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										06-Redis
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										01. Redis基础
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										02. Redis持久化
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										03. Redis主从架构
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										04. Redis分片集群
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										07-消息队列
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										01. Kafka基础
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										02. 使用Kafka会考虑什么问题
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										08-MySQL
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										01. MySQL索引
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										02. MySQL事务和锁机制和MVCC
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										03. MySQL调优
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										09-项目场景类
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										01. 如何实现去重和幂等
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										02. 系统需求多变如何设计
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										03. 设计模式
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										10-计算机网络
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										01. HTTP
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										02.TCP三次握手和四次挥手
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										01. 排序算法
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										00.项目介绍
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										01.环境搭建
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										03.工具包
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										04.从一条短信开始
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										05.选择数据库和技术选型
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										06.MySQL环境搭建
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										07.Kafka环境搭建
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	<!-- 引入正文 -->
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		<h1 id="article-title">

	04. 反射和动态代理
</h1>
<div class="article-meta">
	
	<span>3y</span>
	<span>2021-08-19 11:14:07</span>
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<div id="article-content">
	<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>今天要不来聊聊Java反射？你对Java反射了解多少？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯，Java反射在JavaSE基础中还是很重要的。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：简单来说，反射就是Java可以给我们在运行时获取类的信息</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在初学的时候可能看不懂、又或是学不太会反射，因为初学的时候往往给的例子都是用反射创建对象，用反射去获取对象上的方法/属性什么的，感觉没多大用</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：但毕竟我已经不是以前的我了，跟以前的看法就不一样了。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：理解反射重点就在于理解什么是「运行时」，为什么我们要在「运行时」获取类的信息</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在当时学注解的时候，我们可以发现注解的生命周期有三个枚举值（当时我还告诉过<span style="color:#ab4642"><strong>面试官</strong></span>你呢~）</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：分别是SOURCE、CLASS和RUNTIME，其实一样的，RUNTIME就是对标着运行时</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：我们都知道：我们在编译器写的代码是 .java 文件，经过javac 编译会变成 .class 文件，class 文件会被JVM装载运行（这里就是真正运行着我们所写的代码（虽然是被编译过的），也就所谓的运行时。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯，<strong>你说了那么多，就讲述了什么是运行时</strong>，还是快点进入重点吧</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在运行时获取类的信息，其实就是为了让我们所写的代码更具有「通用性」和「灵活性」</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：要理解反射，需要抛开我们日常写的业务代码。以更高的维度或者说是抽象的思维去看待我们所写的“工具”</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：所谓的“工具”：在单个系统使用叫做“Utils”、被多个系统使用打成jar包叫做“组件”、组件继续发展壮大就叫做“框架”</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtt06i4eg6j60yq09ejs902.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：一个好用的“工具”是需要兼容各种情况的。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：你肯定是不知道用该“工具“的用户传入的是什么对象，但你需要帮他们得到需要的结果。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：例如SpringMVC 你在方法上写上对象，传入的参数就会帮你封装到对象上</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：Mybatis可以让我们只写接口，不写实现类，就可以执行SQL</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：你在类上加上@Component注解，Spring就帮你创建对象</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：这些统统都有反射的身影：约定大于配置，配置大于硬编码。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：通过”约定“使用姿势，使用反射在运行时获取相应的信息（毕竟作为一个”工具“是真的不知道你是怎么用的），实现代码功能的「通用性」和「灵活性」</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯，明白了</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：结合之前说的泛型，想问下：<strong>你应该知道泛型是会擦除的，那为什么反射能获取到泛型的信息呢？</strong></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：我再补充一下：泛型的信息只存在编译阶段，在class字节码就看不到泛型的信息了。那为什么下面这段代码能获取得到泛型的信息呢？</p>
<pre><code class="java">// 抽象类，定义泛型&lt;T&gt;
public abstract class BaseDao&lt;T&gt; &#123;
    public BaseDao()&#123;
        Class clazz = this.getClass();
        ParameterizedType  pt = (ParameterizedType) clazz.getGenericSuperclass(); 
        clazz = (Class) pt.getActualTypeArguments()[0];
        System.out.println(clazz);
    &#125;
&#125;

// 实现类
public class UserDao extends BaseDao&lt;User&gt; &#123;
    public static void main(String[] args) &#123;
        BaseDao&lt;User&gt; userDao = new UserDao();

    &#125;
&#125;
// 执行结果输出
class com.entity.User
</code></pre>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯，这个问题我在学习的时候也想过</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：其实是这样的，可以理解为泛型擦除是有范围的，定义在类上的泛型信息是不会被擦除的。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：Java 编译器仍在 class 文件以 Signature 属性的方式保留了泛型信息</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：Type作为顶级接口，Type下还有几种类型，比如TypeVariable、ParameterizedType、WildCardType、GenericArrayType、以及Class。通过这些接口我们就可以在运行时获取泛型相关的信息。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：好，了解</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>你了解动态代理吗？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯，了解的。动态代理其实就是代理模式的一种，代理模式是设计模式之一。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：代理模型有静态代理和动态代理。静态代理需要自己写代理类，实现对应的接口，比较麻烦。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在Java中，动态代理常见的又有两种实现方式：JDK动态代理和CGLIB代理</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：JDK动态代理其实就是运用了反射的机制，而CGLIB代理则用的是利用ASM框架，通过修改其字节码生成子类来处理。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：JDK动态代理会帮我们实现接口的方法，通过invokeHandler对所需要的方法进行增强。</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtt07mywyqj60t40eiwff02.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：动态代理这一技术在实际或者框架原理中是非常常见的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：像上面所讲的Mybatis不用写实现类，只写接口就可以执行SQL，又或是SpringAOP 等等好用的技术，实际上用的就是动态代理。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯，了解了。</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtlxszoiv5j61400mign202.jpg"></p>
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